Omnivores like the rat must balance their diet from a variety of food sources. It is well documented that rats can select appropriate amounts of fat, protein, and carbohydrate when given pure macronutrient sources, but the mechanism by which they do so has not been firmly established. This is particuarly true for fat and protein; it is known that rats have taste receptors for carbohydrates and can learn to associate the postingestive effects of carbohydrates with flavor cues. The present proposal will test the hypothesis that rats can also learn to associate flavor cues with the postingestive effects of fats and proteins. If so, this may be one important means by which animals regulate their macronutrient intake. A conditioned flavor preference paradigm will be used in two experiments. The rats will be given access to an arbitrarily flavored solution (CS+) that will be paired with intragastric infusions of a protein solution or a fat emulsion: carbohydrate infusions will also be used to demonstrate the effectiveness of the conditioning paradigm. A different flavored solution (CS-) will be paired with intragastric water infusions. Flavor conditioning will be demonstrated using a two-choice preference test with both CS+ and CS- flavors. In one experiment a short-term (one conditioning trial/day) procedure will be used. In a second experiment a long-term training procedure will be used: the rats will have 23 hr/day access to the CS+ or CS- solution and will be automatically infused with the nutrient (fat or protein) or water as they drink the CS solution. If these training procedures prove successful then follow-up experiments will be conducted to further examine the conditioning of macronutrient appetites in the rat.